Why did Romeo write a suicide note to his father in Romeo and Juliet?

Romeo was going through so much that no one but a few
people knew about, and only Juliet could really understand.  His friends and family
(save Friar Laurence) knew about his unrequitted love for Rosaline, but hadn't a clue
that he was in love with another woman.

Leaving a suicide
note would do a couple of things.  First, it would explain the reasoning behind his
decision.  If they just found him dead by his own hand with no explanation, they
might think it was over Rosaline.  He most likely wanted them to know the full
truth.

A second reason for leaving the note would be to
show his parents what the family feud has done; not only the lives taken by the enemies'
swords, but the lives torn apart by being forced to keep secrets that would otherwise
have led to them being disowned by their own families.

A
suicide note to his parents was important for Romeo.  He killed himself because of his
deep grief over Juliet's supposed death, but he didn't want the reasons behind his death
to go unnoticed.